CHAPTER XXI 



THE DEVELOPMENT OF THE FROG'S EGG 



ALL living matter has the capacity of increasing itself 

 under proper conditions to an almost unlimited extent, 

 the food which animals devour being the material out of 

 which the new living substance is made. This living sub- 

 stance exists in isolated particles, or masses, which we call 

 individual animals or plants. The animal or plant has 

 also at any stage a definite form which is not exactly alike 

 in any two individuals, but is roughly alike inside the 

 "species." Now the number of individuals of any species 

 tends constantly to diminish through death. It is actually 

 maintained by reproduction. Reproduction always takes 

 place in one way; namely, by a piece of the parent 

 individual being cut off to form a new individual. This 

 piece may be at first almost shapeless or approximately 

 spherical. Bat as it grows larger it assumes more and 

 more the form and complex structure of the adult. This 

 process of growing into the adult form is development. 

 In most of the more familiar cases development begins 

 with an approximately spherical egg. 



In the case of the frog, the egg is between one and two 

 millimetres in diameter. The eggs, which are numerous, 

 are laid in a common jelly and during development float 

 near the surface of the water in which they are laid. 



332 



